A. It's usually not a matter of choice. In most cases, the load
capacitance is not from a capacitor you've added intentionally;
most often it's an unwanted parasitic, such as the capacitance
of a length of coaxial cable. However, situations do arise where
it's desirable to decouple a dc voltage at the output of an op
amp-for example,when an op amp is used to invert a reference
voltage and drive a dynamic load. In this case, you might want
to place bypass capacitors directly on the output of an op amp.
Either way, a capacitive load affects the op amp's performance.

Q. How does capacitive loading affect op amp performance?

A. To put it simply, it can turn your amplifier into an oscillator.
Here's how:

Op amps have an inherent output resistance, Ro, which, in
conjunction with a capacitive load, forms an additional pole in
the amplifier's transfer function. As the Bode plot shows, at
each pole the amplitude slope becomes more negative by 20 dB/
decade. Notice how each pole adds as much as -90° of phase
shift. We can view instability from either of two perspectives.
Looking at amplitude response on the log plot,circuit instability
occurs when the sum of open-loop gain and feedback
attenuation is greater than unity. Similarly, looking at phase
response, an op amp will tend to oscillate at a frequency where
loop phase shift exceeds -180°, if this frequency is below the
closed-loop bandwidth. The closed-loop bandwidth of a
voltage-feedback op amp circuit is equal to the op amp's
bandwidth product (GBP, or unity-gain frequency), divided
by the circuit's closed loop gain (ACL).

Phase margin of an op amp circuit can be thought of as the
amount of additional phase shift at the closed loop bandwidth
required to make the circuit unstable (i.e., phase shift + phase
margin = -180°). As phase margin approaches zero, the loop
phase shift approaches -180° and the op amp circuit approaches
instability. Typically, values of phase margin much less than
45° can cause problems such as "peaking" in frequency
response, and overshoot or "ringing" in step response. In order
to maintain conservative phase margin, the pole generated by
capacitive loading should be at least a decade above the circuit's
closed loop bandwidth.When it is not, consider the possibility
of instability.

Q. So how do I deal with a capacitive load?

A. First of all you should determine whether the op amp can safely
drive the load on its own. Many op amp data sheets specify a
"capacitive load drive capability". Others provide typical data
on "small-signal overshoot vs. capacitive load". In looking at
these figures, you'll see that the overshoot increases
exponentially with added load capacitance. As it approaches
100%, the op amp approaches instability. If possible, keep it
well away from this limit. Also notice that this graph is for a
specified gain. For a voltage feedback op amp, capacitive load
drive capability increases proportionally with gain. So aVF op
amp that can safely drive a 100-pF capacitance at unity gain
should be able to drive a 1000-pF capacitance at a gain of 10.

A few op amp data sheets specify the open loop output
resistance (Ro), from which you can calculate the frequency of gain-the added pole as described above.The circuit will be stable if
the frequency of the added pole (fP) is more than a decade
above the circuit's bandwidth.

If the op amp's data sheet doesn't specify capacitive load drive
or open loop output resistance, and has no graph of overshoot
versus capacitive load, then to assure stability you must assume
that any load capacitance will require some sort of compensa-tion
technique.There are many approaches to stabilizing standard
op amp circuits to drive capacitive loads. Here are a few:

Noise-gain manipulation: A powerful way to maintain
stability in low-frequency applications-often overlooked by
designers-involves increasing the circuit's closed-loop gain
(a/k/a "noise gain") without changing signal gain,thus reducing
the frequency at which the product of open-loop gain and
feedback attenuation goes to unity. Some circuits to achieve
this, by connecting RD between the op amp inputs, are shown
below. The "noise gain" of these circuits can be arrived at by
the given equation.

Since stability is governed by noise gain rather than by signal
gain, the above circuits allow increased stability without
affecting signal gain. Simply keep the "noise bandwidth"
(GBP/ANOISE) at least a decade below the load generated pole
to guarantee stability.

One disadvantage of this method of stabilization is the
additional output noise and offset voltage caused by increased
amplification of input-referred voltage noise and input offset
voltage. The added dc offset can be eliminated by including
CD in series with RD, but the added noise is inherent with this technique. The effective noise gain of these circuits with and
without CD are shown in the figure.

CD, when used, should be as large as feasible; its minimum
value should be 10 ANOISE/(2 pRDGBP) to keep the "noise pole" at least a decade below the "noise bandwidth".

Out-of-loop compensation: Another way to stabilize an op
amp for capacitive load drive is by adding a resistor, RX,
between the op amp's output terminal and the load capacitance,
as shown below.Though apparently outside the feedback loop,
it acts with the load capacitor to introduce a zero into the
transfer function of the feedback network, thereby reducing
the loop phase shift at high frequencies.

To ensure stability, the value of RX should be such that the
added zero (fZ) is at least a decade below the closed loop
bandwidth of the op amp circuit.With the addition of RX,circuit
performance will not suffer the increased output noise of the
first method, but the output impedance as seen by the load
will increase.This can decrease signal gain, due to the resistor
divider formed by RX and RL. If RL is known and reasonably
constant, the results of gain loss can be offset by increasing the
gain of the op amp circuit.

This method is very effective in driving transmission lines.The
values of RL and RX must equal the characteristic impedance
of the cable (often 50ohms or 75ohms) in order to avoid standing
waves. So RX is pre-determined, and all that remains is to
double the gain of the amplifier in order to offset the signal
loss from the resistor divider. Problem solved.

In-loop compensation: If RL is either unknown or dynamic,
the effective output resistance of the gain stage must be kept
low. In this circumstance, it may be useful to connect RX inside
the overall feedback loop, as shown below. With this
configuration, dc and low-frequency feedback comes from the
load itself, allowing the signal gain from input to load to remain
unaffected by the voltage divider, RX and RL.

The added capacitor, CF, in this circuit allows cancellation of
the pole and zero contributed by CL.To put it simply, the zero
from CF is coincident with the pole from CL, and the pole
from CF with the zero from CL.Therefore, the overall transfer
function and phase response are exactly as if there were no
capacitance at all. In order to assure cancellation of both pole/
zero combinations, the above equations must be solved
accurately. Also note the conditions; they are easily met if the
load resistance is relatively large.

Calculation is difficult when RO is unknown. In this case, the
design procedure turns into a guessing game-and a
prototyping nightmare.A word of caution about SPICE:SPICE
models of op amps don't accurately model open-loop output
resistance (RO); so they cannot fully replace empirical design
of the compensation network.

It is also important to note that CL must be of a known (and
constant) value in order for this technique to be applicable. In
many applications, the amplifier is driving a load "outside the
box," and CL can vary significantly from one load to the next.
It is best to use the above circuit only when CL is part of a
closed system.

One such application involves the buffering or inverting of a
reference voltage, driving a large decoupling capacitor. Here,
CL is a fixed value, allowing accurate cancellation of pole/zero
combinations. The low dc output impedance and low noise of
this method (compared to the previous two) can be very
beneficial. Furthermore, the large amount of capacitance likely
to decouple a reference voltage (often many microfarads) is
impractical to compensate by any other method.

All three of the above compensation techniques have advantages
and disadvantages. You should know enough by now to decide
which is best for your application. All three are intended to be
applied to "standard", unity gain stable, voltage feedback op
amps. Read on to find out about some techniques using special
purpose amplifiers.

Q. My op amp has a "compensation" pin. Can I overcompensate the
op amp so that it will remain stable when driving a capacitive load?

A. Yes. This is the easiest way of all to compensate for load
capacitance. Most op amps today are internally compensated
for unity-gain stability and therefore do not offer the option to
"overcompensate". But many devices still exist with inherent
stability only at very high noise gains. These op amps have a
pin to which an external capacitor can be connected in order
to reduce the frequency of the dominant pole. To operate stably
at lower gains, increased capacitance must be tied to this pin
to reduce the gain-bandwidth product. When a capacitive load
must be driven, a further increase (overcompensation) can
increase stability—but at the expense of bandwidth.

Q. So far you've only discussed voltage feedback op amps exclusively,
right? Do current feedback (CF) op amps behave similarly with
capacitive loading? Can I use any of the compensation techniques
discussed here?

A. Some characteristics of current feedback architectures
require special attention when driving capacitive loads, but
the overall effect on the circuit is the same. The added pole,
in conjunction with op-amp output resistance, increases
phase shift and reduces phase margin, potentially causing
peaking, ringing, or even oscillation. However, since a CF
op amp can't be said to have a "gain-bandwidth product"
(bandwidth is much less dependent on gain), stability can't
be substantially increased simply by increasing the noise
gain. This makes the first method impractical. Also, a
capacitor (CF) should NEVER be put in the feedback loop
of a CF op amp, nullifying the third method. The most direct
way to compensate a current feedback op amp to drive a
capacitive load is the addition of an "out of loop" series
resistor at the amplifier output as in method 2.

Part Number

Ch

BWMHz

SRV/ms

vnnV/Hz

infA/Hz

VOSmV

IbnA

SupplyVoltageRange[V]

IQmA

ROohms

CapLoadDrive[pF]

Notes

AD817

1

50

350

15

1500

0.5

3000

5-36

7

8

unlim

AD826

2

50

350

15

1500

0.5

3000

5-36

6.8

8

unlim

AD827

2

50

300

15

1500

0.5

3000

9-36

5.25

15

unlim

AD847

1

50

300

15

1500

0.5

3000

9-36

4.8

15

unlim

AD848

1

35

200

5

1500

0.5

3000

9-36

5.1

15

unlim

GMIN=5

AD849

1

29

200

3

1500

0.3

3000

9-36

5.1

15

unlim

GMIN=25

AD704

4

0.8

0.15

15

50

0.03

0.1

4-36

0.375

10000

AD705

1

0.8

0.15

15

50

0.03

0.06

4-36

0.38

10000

AD706

2

0.8

0.15

15

50

0.03

0.05

4-36

0.375

10000

OP97

1

0.9

0.2

14

20

0.03

0.03

4-40

0.38

10000

OP279

2

5

3

22

1000

4

300

4.5-12

2

22

10000

OP400

4

0.5

0.15

11

600

0.08

0.75

6-40

0.6

10000

AD549

1

1

3

35

0.22

0.5

0.00015

10-36

0.6

4000

OP200

2

0.5

0.15

11

400

0.08

0.1

6-40

0.57

2000

OP467

4

28

170

6

8000

0.2

150

9-36

2

1600

AD744

1

13

75

16

10

0.3

0.03

9-36

3.5

1000

comp.term

AD8013

3

140

1000

3.5

12000

2

3000

4.5-13

3.4

1000

current fb

AD8532

2

3

5

30

50

25

0.005

3-6

1.4

1000

AD8534

4

3

5

30

50

25

0.005

3-6

1.4

1000

OP27

1

8

2.8

3.2

1700

0.03

15

8-44

6.7

70

1000

OP37

1

12

17

3.2

1700

0.03

15

8-44

6.7

70

1000

GMIN=5

OP270

2

5

2.4

3.2

1100

0.05

15

9-36

2

1000

OP470

4

6

2

3.2

1700

0.4

25

9-36

2.25

1000

OP275

2

9

22

6

1500

1

100

9-44

2

1000

OP184

1

4.25

4

3.9

400

0.18

80

4-36

2

1000

OP284

2

4.25

4

3.9

400

0.18

80

4-36

2

1000

OP484

4

4.25

4

3.9

400

0.25

80

4-36

2

1000

OP193

1

0.04

15

65

50

0.15

20

3-36

0.03

1000

OP293

2

0.04

15

65

50

0.25

20

3-36

0.03

1000

Q. This has been informative, but I'd rather not deal with any of these
equations. Besides, my board is already laid out, and I don't want
to scrap this production run. Are there any op amps that are
inherently stable when driving capacitive loads?

A. Yes. Analog Devices makes a handful of op amps that drive
"unlimited" load capacitance while retaining excellent phase
margin. They are listed in the table, along with some other op
amps that can drive capacitive loads up to specified values.
About the "unlimited" cap load drive devices: don't expect to
get the same slew rate when driving 10 µF as you do when
driving purely resistive loads. Read the data sheets for details.